Solidification microstructure-dependent hydrogen generation behavior of Al–Sn and Al–Fe alloys in alkaline medium

This work deals with the development of quantitative correlations of hydrogen evolution performance with solidification microstructural and thermal parameters in Al–1Sn, Al–2Sn, Al–1Fe, and Al-1.5Fe [wt.%] alloys. The cellular growth as a function of growth and cooling rates is evaluated using power type experimental laws, which allow determining representative intervals of microstructure length scale for comparison purposes with the results of immersion tests in 5 wt%NaOH solution. For both Al alloys systems, hydrogen evolution becomes slower as the alloy solute content increased. However, for a given alloy composition, whereas a more homogeneous distribution of Sn-rich particles promotes faster hydrogen generation using Al–Sn alloys, coarsening of Al₆Fe IMCs (intermetallic compounds) fibers favors hydrogen production using Al–Fe alloys. When solidification conditions that result in a range of cellular spacings within 16 and 19 μm are considered, the specific hydrogen production of the Al-1wt.%Fe alloy is higher than that of the other studied alloys.     

时效温度对非平衡凝固K424合金析出相及硬度的影响

采用铜模喷铸与等温时效相结合的方法,研究了快冷K424高温合金的非平衡凝固组织及其沉淀相析出行为。结果表明,铜模喷铸合金组织发生了显著细化,二次枝晶间距相比铸态大幅降低,溶质偏析的减弱有利于抑制粗大碳化物及共晶组织形成。随铜模内径下降,枝晶形貌由无方向性等轴晶逐渐转变为择优生长的定向枝晶。经800~1000℃等温时效30min后,快冷合金的过饱和固溶体组织中开始析出γ′相。随时效温度提高,沉淀相尺寸及体积分数同时增加。900℃×30min时效后可获得大量细小均匀的球形γ′相,快冷合金的显微硬度(HV)高达478,相比原始非平衡态合金提高了13%。     

Reduction in Microsegregation in Al-Cu Alloy by Alternating Magnetic Field

The microsegregation behavior of the Al-4.5 wt%Cu alloy solidified at different cooling rates under the alternating magnetic field(AMF) was investigated.The experimental results showed that the amount of non-equilibrium eutectics in the interdendritic region decreased upon applying the AMF at the same cooling rate.The change in microsegregation could be explained quantificationally by the modifications of dendritic coarsening,solid-state back diffusion and convection in the AMF.The enhanced diffusivity in the solid owing to the AMF was beneficial for the improvement in microsegregation compared to the cases without an AMF.In contrast,the enhanced dendritic coarsening and forced convection in the AMF were found to aggravate the microsegregation level.Considering the contributions of the changes in above factors,an increase in solid diffusivity was found to be primarily responsible for the reduced microsegregation in the AMF.In addition,the microsegregation in the AMF was modeled using the analytical model developed by Voller.The calculated and experimental results were in reasonable agreement.     

Effect of sintering temperature on solidification and migration of non-volatile and volatile heavy metals in glass-ceramic

Using tailings as material to prepare glass-ceramic is an excellent way to achieve the resource utilization of solid waste. However, at present, researches on the solidification and migration of heavy metals are limited. Therefore, in this study, ten groups of samples were prepared by controlling sintering temperatures. The solidification, migration, and leaching behavior of non-volatile and volatile heavy metals were studied. The research showed that, with the increase of temperature, the properties of the samples were improved. Fe participated in the phase transformation and evolved into insoluble iron pyroxenes solid solution, while Pb was homogeneously distributed in the glass matrix of glass-ceramics. The leaching concentrations of Fe and Pb in the glass-ceramics were 0.055 mg/L ～0.087 mg/L and 0.074 mg/L ～0.140 mg/L, which were far below the threshold value. The results showed that heavy metals can be effectively solidified in glass-ceramics and have good environmental benefits.     

压力铸渗法制备金属基复合材料

复合材料其有高强度、硬度以及良好的耐磨性等优良的性能。在矿山机械、航灭、交通、结构材料等领域具有广阔的应用前景。材料研究工作者对金属基复合材料的研究日益深入和广泛，因此，制备金属基复合材料的工艺技术也得到了迅速的发展。压力铸渗法是一种非常有效的方法，在这种工艺技术中．液态金属在压力作用下渗入到增强体颗粒或纤维制成的预制型中，冷却后即可得到分布均匀的复合材料。利用这种工艺制备的复合材料不仅能满足使用要求，而且可以得到净成型工件。在本文中主要针对陶瓷颗粒预制型的压力铸渗的研究进行阐述。     

陈腐生活垃圾固化体的路用性能及其重金属稳定性研究

目前，生活垃圾处理技术仍以填埋为主，而填埋处理技术又面临着选址、年处理垃圾量增加和很多填埋场填埋年限到期的问题。受循环经济启发，将陈腐5年以上的填埋场生活垃圾，加入固化剂及工业废渣进行固化处理，可以解决填埋场内陈腐垃圾的出路问题，而制成的固化体强度及耐久性还可达到路用地基材料的技术要求，同时符合环境排放要求，不会带来二次污染，文章探讨了陈腐垃圾固化体重金属的稳定性问题。     

Large-scale parallel lattice Boltzmann–cellular automaton model of two-dimensional dendritic growth

An extremely scalable lattice Boltzmann (LB)–cellular automaton (CA) model for simulations of two-dimensional (2D) dendritic solidification under forced convection is presented. The model incorporates effects of phase change, solute diffusion, melt convection, and heat transport. The LB model represents the diffusion, convection, and heat transfer phenomena. The dendrite growth is driven by a difference between actual and equilibrium liquid composition at the solid–liquid interface. The CA technique is deployed to track the new interface cells. The computer program was parallelized using the Message Passing Interface (MPI) technique. Parallel scaling of the algorithm was studied and major scalability bottlenecks were identified. Efficiency loss attributable to the high memory bandwidth requirement of the algorithm was observed when using multiple cores per processor. Parallel writing of the output variables of interest was implemented in the binary Hierarchical Data Format 5 (HDF5) to improve the output performance, and to simplify visualization. Calculations were carried out in single precision arithmetic without significant loss in accuracy, resulting in 50% reduction of memory and computational time requirements. The presented solidification model shows a very good scalability up to centimeter size domains, including more than ten million of dendrites.     

Corrosion of Al—Fe in hydrochloric acid

Abstract

Nanoprocessing can be considered a distinct form of grain boundary engineering by which property enhancements are achieved by deliberately increasing the volume fractions of grain boundaries and triple junctions in a material. Electrodeposition has been shown to be a technologically viable production method to synthesize such materials both in bulk form and as thin films. The mechanical, magnetic, electrical and corrosion properties exhibited by nanocrystalline metals produced by this method make them strong contenders for a number of advanced materials applications.

Résumé

Nous pouvons considérer le nanotraitement comme une forme distincte de génie des joints de grains par lequel nous obtenons des améliorations de propriétés en augmentant délibérément le volume des fractions des joints de grains et des jonctions triples dans les matériaux. On a montré que l'électrodéposition est une méthode technologiquement viable pour synthétiser de tels matériaux, aussi bien en films minces que sous forme massive. Les propriétés mécaniques, magnétiques, électriques et corrosives des métaux nanocristallins obtenus par cette méthode, font d'eux des concurrents sérieux pour nombreuses applications de matériaux avancés.     

Investigation of crystallisation kinetics of Bi0·5Se99·5-xZnx glasses by differential scanning calorimetry

Abstract

The kinetics of crystallisation in Bi_0·5Se_99·5-xZn_x (x=0, 0·1, 0·2, 0·5, 1·0) glasses are studied by non-isothermal method using differential scanning calorimetry. Differential thermal analysis was performed at different heating rates of 5, 10, 15, and 20 K min^-1. The values of glass transition and crystallisation temperatures are found to be composition and heating rate dependent. From the heating rate dependence of the glass transition and crystallisation temperatures, the activation energy of crystallisation ΔE _c, the order parameter n and the enthalpy released ΔH _c are calculated. The thermal stability (T _c–T _g) was found to be maximum for Bi_0·5Se_99·5 glass, which suggest that this glass can be considered as a critical composition at which the system becomes chemically ordered. The crystallisation enthalpy ΔH _c is maximum for Bi_0·5Se₉₉Zn_0·5 glass, hence it is the least stable glass in the present system.     

Prediction of solidification behaviour and microstructure of Ni based alloys obtained by casting and direct additive laser growth

Numerical tools are now used widely in the prediction of material properties necessary in order to gain a better understanding of the relationship between material properties and performance, to improve the reliability of processes and the quality of the final product, and to reduce costs, waste and energy use. In this paper, the solidification properties and the microstructure of some commercial Ni based alloys were analysed and predicted numerically using the ProCAST software. The microstructure of a sample obtained by the direct additive laser growth, a new additive manufacturing technique based on the selective laser melting, is presented and discussed. Numerical approaches and software packages that can be used to model additive manufacturing processes are discussed and critically analysed.